As an anesthetic needle used to administer spinal anesthesia, a compound needle including outer and inner needles as disclosed in Patent Document 1 below (Japanese Unexamined Utility Model Registration Application Publication No. 6-44549) is known. With this compound needle, the outer needle is inserted in order to cause a distal end thereof to reach the epidural space. After that, the inner needle is inserted into the outer needle so as to cause a distal end thereof to protrude from the distal end of the outer needle, to penetrate through the dura mater, and to reach the subarachnoidal space. Then, spinal anesthesia is administered by injecting an anesthetic solution through the inner needle into the subarachnoidal space. In this spinal anesthesia, during a confirming operation after the inner needle has penetrated through the dura mater or during a medicine injecting operation, the inner needle may be advanced or retracted. This may cause the distal end of the inner needle to move out of the dura mater and result in failure of spinal anesthesia, or may cause the distal end of the inner needle to be excessively inserted and damage nerve tissue. For this reason, it is desirable that the inner needle can be fixed so as not to be advanced or retracted relative to the outer needle. Patent Document 2 below (Japanese Unexamined Patent Application Publication No. 10-305101) discloses a compound needle, an inner needle of which can be fixed as described above. In this compound needle, a base portion of a proximal end of an outer needle and a proximal end portion of a proximal end of an inner needle are threadably engaged with each other. The inner needle, when rotated, can be advanced or retracted relative to the outer needle, and when not rotated, fixed relative to the outer needle.
Although a mechanism disclosed in Patent Document 2 above in which the proximal end portions of the inner and outer needles are threadably engaged with each other can fix the inner needle relative to the outer needle, the inner needle needs to be rotated in order to be advanced or retracted. Thus, operation is very complex. In addition, there is a significant disadvantage that insertion of the inner needle into the dura mater cannot be sensed.
Patent Document 3 below (Japanese Unexamined Patent Application Publication No. 2002-306596) discloses a technology that overcomes the above-described problems. With this technology, by rotating an inner needle by 180° or a smaller angle relative to an outer needle, an unlocked state, in which the inner needle can be advanced or retracted relative to the outer needle, and a locked state, in which the inner needle cannot be advanced or retracted relative to the outer needle, can be switched. In the unlocked state, the inner needle can be straightly advanced relative to the outer needle without resistance. Thus, a moment when the distal end of the inner needle is inserted into the dura mater can be clearly sensed.    Patent Document 1: Japanese Unexamined Utility Model Registration Application Publication No. 6-44549    Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-305101    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-306596
With the technology disclosed in Patent Document 3 above, the amount of rotation of the inner needle can be markedly reduced compared to that in the technology disclosed in Patent Document 2 above, and accordingly, can cause the moment when the distal end of the inner needle is inserted into the dura mater to be clearly sensed due to insertion resistance. However, the locked and unlocked states are switched by rotating the inner needle. Some anesthesiologists suggest that it is unavoidable that the internal needle, when it is rotated, is slightly advanced or retracted. Thus, the anesthesiologists need to pay close attention while administering anesthesia. Still, in many cases, there is a possibility of damaging nerve tissue due to advancement of the inner needle and a possibility of failure of spinal anesthesia caused by removal of the distal end of the inner needle from the dura mater due to retraction of the inner needle.
An object of the present invention is to develop an anesthetic compound needle that allows an unlocked state, in which the inner needle can be advanced or retracted relative to the outer needle, and a locked state, in which the inner needle cannot be advanced or retracted relative to the outer needle, to be switched by a simple operation without rotating the inner needle. This arrangement allows the moment when the distal end of the inner needle is inserted into the dura mater to be clearly sensed.